In addition to circuit-switched speech and data, an increasing need exists in cellular radio networks to transfer packet-switched data, wherein user data is transferred as packets. New solutions have been developed to enable the existing cellular radio networks to be changed so as to enable packet transfer to take place as cost-efficiently and effectively as possible.
GPRS (General Packet Radio Service) is a GSM-based service wherein capacity free from air interface circuit switching is used for packet transfer. In basic GPRS, GMSK (Gaussian Minimum-Shift Keying) is used as a modulation method.
EGPRS (Enhanced GPRS) uses EDGE (Enhanced Data Rates for GSM Evolution) technique for increasing data transmission capacity. In addition to the GMSK modulation normally used in the GSM, 8-PSK (8-Phase Shift Keying) modulation is also used. The aim is to provide real-time services using packet switching for transferring speech and video image, for example. In principle, the data transmission capacity may vary from few kilo-bits per second up to 400 kbit/s.
Other methods are also used to increase capacity, including incremental redundancy, blind detection of modulation and link adaptation.
In incremental redundancy, a receiver is equipped with memory in which the bits of erroneously received radio blocks are stored. The retransmitted radio blocks are then combined with the stored radio blocks, which enables the radio block to be decoded. An example of such a protocol is type 11 hybrid ARQ (Automatic Repeat Request).
Blind detection of modulation means that it is unnecessary to signal to the receiver which modulation method is used but the receiver detects it while receiving the signal.
Link adaptation refers to changing modulation and channel coding based on measurements carried out in the channel. The aim is to optimize. the use of the radio resource, taking temporary variations in the conditions of the radio interface into account. The aim is to optimize throughput of user data and minimize delays. For example, coding can be reduced under desirable conditions, which means that a larger amount of user payload can be transferred. Similarly, one modulation method may be more suitable than the other for certain conditions on the radio interface. Different combinations of modulation and channel coding may be called MCS (Modulation and Coding Schemes). The radio interface is also called an air interface or Um interface.
However, data transfer on an Abis interface between a base station and a base station controller remains a bottleneck. According to the prior art, a certain number of transmission channels is allocated fixedly to base-station-specific operations and maintenance signalling. Similarly, a certain number of transmission channels is allocated fixedly to transceiver-specific telecommunication signalling. The telecommunication signalling herein refers specifically to signalling concerning Um interface connections, such as connection establishment, handover and measurement reports.
The MCS selected greatly affects the amount of packet data transferred on the Um interface. Consequently, the amount of packet data transferred on the Abis interface may vary greatly.
The current solution is to allocate the user-specific Abis interface capacity to packet data according to the MCS which requires the highest capacity. In EGPRS, for example, a channel providing at least 64 kbit/s is then allocated fixedly. A problem presenting itself is that when an MCS necessitating a low data transmission capacity is used, Abis interface transmission capacity is being wasted since it is dimensioned according to the highest transfer need.